Conventionally, in order to raise the flexibility of base station installation in radio communication systems, particularly mobile communication systems, configurations in which the functions of a base station are distributed between two apparatuses, namely, a BBU (Base Band Unit) and an RRH (Remote Radio Head), and the BBU and RRH are physically separated, have been considered. As one mode for functional splitting schemes between a BBU and an RRH, a functional splitting scheme in which the functions of the MAC (Media Access Control) layer and higher are performed by a BBU and the functions of the physical layer are performed by RRHs, as shown in FIG. 17, has been considered (see, e.g., Non-Patent Document 1). This functional splitting scheme is called a MAC-PHY splitting scheme or an L2 C-RAN (Layer 2 Centralized/Cloud-Radio Access Network) scheme.
On the other hand, as a mode of schemes for splitting the functions between a BBU and an RRH, a functional splitting scheme in which the functions of the MAC (Media Access Control) layer and higher and the coding functions, which are a part of the physical layer functions, are performed by a BBU, and the functions of the physical layer other than the coding functions are performed by RRHs, as shown in FIG. 18, has been considered (see, e.g., Non-Patent Document 2). This functional splitting scheme is called the SPP (Split-PHY Processing) scheme.
As schemes for demodulating radio signals received in a base station or a terminal apparatus, there are soft-decision demodulation schemes in which, instead of outputting signal bits obtained by demodulation as bit values 0 or 1, the signal bits are output as real-value ratios called likelihoods, indicating the probability that a signal bit is 0 or 1 (see, e.g., Non-Patent Document 3). In a soft-decision demodulation scheme, the output obtained by demodulation is called the LLR (Log Likelihood Ratio). In general, the larger the LLR value is in the positive direction, the higher the probability that the signal bit is 1, and the lower the value is in the negative direction (i.e., the higher the absolute value), the higher the probability that the signal bit is 0.
Additionally, in a mobile communication system, the area covered by a single RRH is referred to as a cell, and in general, the coverage areas of multiple adjacent cells overlap. For this reason, when a terminal apparatus is located near a cell edge, there is a problem in that the radio signals being exchanged between the terminal apparatus and a desired RRH can encounter interference from radio signals exchanged between the terminal apparatus and the RRH of an adjacent cell, thereby significantly reducing the radio transmission rate. As a means for solving such a problem, CoMP (Coordinated Multi-Point transmission/reception) technology, in which adjacent RRHs cooperate with each other to communicate with a terminal apparatus located near the cell edges, as shown, for example, in FIG. 19, has been considered (see, e.g., Non-Patent Document 4).
In FIG. 19, there are two cooperating RRHs, but there may be two or more RRHs. The possibility of installing RRHs at a high density and having multiple RRHs constantly performing CoMP with respect to multiple terminal apparatuses, regardless of whether or not the terminal apparatuses are located at the cell edges, thereby increasing the system capacity, has been considered for use in future mobile communication systems. CoMP techniques include a technique known as selective combining, in which, among the reception signals from the multiple cooperating RRHs, the reception signal having the highest reception quality is selected (see, e.g., Non-Patent Document 5). In this case, the reception quality refers, for example, to the received signal power, the received SNR (Signal to Noise Ratio), or the received SINR (Signal to Interference plus Noise Ratio). This selective combining may be applied to CoMP in BBUs and RRHs using a MAC-PHY split functional splitting scheme.
FIG. 20 is a diagram showing an example of a system configuration of a radio communication system 1000 that performs uplink selectively combined signal transmission with a conventional MAC-PHY split. The radio communication system 1000 includes a terminal apparatus 91, multiple RRHs 92-1 and 92-2, and a BBU 93. The RRHs 92-1 and the 92-2 are provided with similar structures, so the RRH 92-1 will be explained as an example.
The RRH 92-1 includes an RF (Radio Frequency) reception unit 921-1, a channel estimation unit 922-1, a demodulation unit 923-1, and a decoding unit 924-1. The BBU 93 includes a selective combining unit 931.
The RF reception unit 921-1 receives signals transmitted from the terminal apparatus 91. Of the received signals, the RF reception unit 921-1 outputs reference signals to the channel estimation unit 922-1, and outputs data signals to the demodulation unit 923-1. The reference signals are signals for extracting channel information regarding the radio transmission path, and include signals that are known between the terminal apparatus and the RRH. The data signal is a signal that is to be transmitted to the BBU, including a series of signal bits.
The channel estimation unit 922-1 estimates the channel information and measures the reception quality on the radio transmission path on the basis of the reference signals output from the RF reception unit 921-1. The channel estimation unit 922-1 outputs the channel information estimation result and the reception quality measurement result to the demodulation unit 923-1.
The demodulation unit 923-1 uses the channel information estimation result and the reception quality measurement result output from the channel estimation unit 922-1 to obtain LLR values (soft decision values) by performing equalization and soft-decision demodulation on the received data signals. The demodulation unit 923-1 outputs the obtained LLR values (soft decision values) and the reception quality measurement result (information on the reception quality) to the decoding unit 924-1.
The decoding unit 924-1 decodes the LLR values output from the demodulation unit 923-1 to restore bit data (hard decision values). It is to be noted that during this decoding step, an error detection code called a CRC (Cyclic Redundancy Check) is used to determine whether or not errors are included in the decoded bit data. Each RRH 92 transmits the decoded bit data and the information on the reception quality (hereinafter referred to as “reception quality information”) measured by the channel estimation unit 922 to the BBU 93.
The selective combining unit 931 of the BBU 93 compares the reception quality information transmitted from each RRH 92, selects the bit data of the RRH 92 having the higher reception quality, and discards the bit data transmitted from the other RRH 92.
Additionally, as one CoMP technique on an uplink (the direction from the RRHs to the BBU), a technique in which an SPP functional splitting scheme is applied, LLRs obtained in the respective RRHs are transmitted to the BBU, and the BBU combines the LLRs obtained by the respective RRHs has been considered (see Non-Patent Document 6).
FIG. 21 is a diagram showing an example of a system configuration of a radio communication system 1000a that performs uplink LLR-combined signal transmission in conventional SPP. The radio communication system 1000a includes a terminal apparatus 91a, multiple RRHs 92a-1 and 92a-2, and a BBU 93a. The RRHs 92a-1 and the 92a-2 are provided with similar structures, so the RRH 92a-1 will be explained as an example.
The RRH 92a-1 includes an RF reception unit 921a-1, a channel estimation unit 922a-1, and a demodulation unit 923a-1. The BBU 93a includes an LLR combining unit 932 and a decoding unit 933.
The RF reception unit 921a-1 receives signals transmitted from the terminal apparatus 91a. Of the received signals, the RF reception unit 921a-1 outputs reference signals to the channel estimation unit 922a-1, and outputs data signals to the demodulation unit 923a-1. The channel estimation unit 922a-1 estimates the channel information and measures the reception quality on the radio transmission path on the basis of the reference signals output from the RF reception unit 921a-1. The channel estimation unit 922a-1 outputs the channel information estimation result and the reception quality measurement result to the demodulation unit 923a-1. The demodulation unit 923a-1 uses the channel information estimation result and the reception quality measurement result output from the channel estimation unit 922a-1 to obtain LLR values (soft decision values) by performing equalization and soft-decision demodulation on the received data signals. The demodulation unit 923a-1 transmits the obtained LLR values (soft decision values) to the BBU 93a. 
The LLR combining unit 932 in the BBU 93a combines the LLR values output from the RRHs 92a and outputs the combined LLR value to the decoding unit 933. The decoding unit 933 decodes the combined LLR value output from the LLR combining unit 932 and thereby obtains signal bit data (hard decision values). The decoding unit 933 outputs the obtained signal bit data.
As described above, with an SPP base station functional splitting scheme, LLR-combined CoMP is used to input more highly reliable LLR values to the decoding unit 933 for decoding, thereby making it possible to decrease the bit errors in the radio signals and improve the radio transmission characteristics.